1. Field of the Invention
The present invention generally relates to an ion beam irradiating apparatus and, more particularly, to an ion beam irradiating apparatus including an ion neutralizer suitable for neutralizing electric charges present on a sample surface, for instance, a surface of a semiconductor substrate having an electric insulating structure.
2. Description of the Related Art
Various types of ion beam neutralizers have been proposed for use in an ion beam irradiating apparatus. Two typical types of conventional ion neutralizers are shown in the illustrations of FIGS. 1A and 1B.
In these illustrations, an electron shower (neutralizer) 1 comprises: a cup-shaped main body 2; a dot-shaped filament 3 provided in the main body 2; and a grating type drawing electrode 4 disposed in an opening section of the mainbody 2. Reference numeral 5 denotes an ion beam, reference numeral 6 is an electron beam, and reference numeral 7 is a sample to be irradiated.
The operation of the typical ion neutralizers will now be described. In the conventional neutralizer shown in FIG. 1A, the electron shower 1 electrically draws the electron beams 6 from the filament 3 at an accelerating voltage of approximately 100V by way of the drawing electrode 4. The electron beams 6 are emitted into a space through which the ion beam 5 passes, and the negative electric charges of the electrons are given to the ion beam 5, thereby neutralizing the same. Alternatively, in the conventional neutralizer shown in FIG. 1B, the electron beams 6 are directly irradiated onto the surface of the sample 7, thereby preventing accumulation of an electric charge in the sample 7 due to irradiating of the ion beam 5 onto the sample 7.
In recent years, an ion beam irradiating technique, for example in using an ion implanting apparatus of a semiconductor, has been put into practical use as a technique to dope impurities into an integrated circuit.
However, when As.sup.+ ions were implanted by a large current into an integrated circuit, e.g., a 1 Mbit DRAM (dynamic random access memory) having a high packaging density, line widths thereof or a thickness of insulating layer of the submicron order, the potentials of the insulating layer in the integrated circuit and of the exposed semiconductor layer or conductive layer surrounded by the insulating layer attain high values due to accumulation of the positive electric charges (hereinafter, referred to as a "positive charging") carried thereto by the incident ion beam. As a result, the subsequent ion beam is deflected and the ions are not uniformly implanted. In the worst case, an electrical insulation breakdown may occur, so that the integrated circuit becomes inoperative. Thus, even if a large current type ion implanting apparatus is used to improve the throughput, an ion current value must be limited to a value such that a typical positive charging phenomenon does not occur in the integrated circuit. A serious practical problem therefore still remains.
To solve this problem, there has been proposed a method whereby the ion implantation is executed after a conductive material was coated onto the surface of an integrated circuit. This conventional method is disclosed in detail in, e.g., JP-A-57-75463, JP-A-58-75463, and JP-A-60-133757 (KOKAI). However, according to these conventional methods, processes to coat the conductive material and to thereafter eliminate it are needed before and after the ion implanting steps. Thus, there are drawbacks such that the whole process to manufacture an integrated circuit become complicated, and accordingly, the yield and throughput deteriorate.
Therefore, as a method of preventing the positive charging on the integrated circuit surface, attention is focussed on a method whereby electrons are supplied into an ion beam or onto the surface of an integrated circuit (hereinafter, simply referred to as a "sample") and the ion beam is thus neutralized.
As a conventional method of neutralizing the ion beam by use of electrons, a method whereby the positive charging is prevented by directly implanting electrons into a sample (e.g., per to JP-A-59-204231) is known. However, according to this prior art method, since an electron beam is also irradiated onto the sample surface on which no ion beam is irradiated, there is another problem such that the negative charging occurs in this region by the electrons and an insulation breakdown occurs in the sample.
On the other hand, in JP-A-57-87056 (corresponding to U.S. patent application Ser. No. 190,297, filed on Sept. 24, 1980), there is disclosed a method whereby the primary electrons are accelerated and drawn and irradiated onto a dummy target and an ion beam is neutralized by the secondary electrons generated from the dummy target. According to this method, there is an one result is that in the energy distribution of the secondary electrons which are emitted from the dummy target, secondary electrons of a low energy of about 10 eV are used. However, this method still has practical problems, such that a sample is adversely influenced by the reflected electrons having the same energy (hundreds of electron volts) as that of the primary electrons, that an amount of secondary electrons which are emitted changes remarkably in dependence on the surface condition of the dummy target, and the like.
Further, the foregoing conventional ion beam neutralizer has the following problems. Namely, a drawing voltage of approximately 100 V is needed to draw an amount of electrons necessary to neutralize the positive electric charges of an ion beam. However, since the electrons have the energy corresponding to the drawing voltage, the electrons are irradiated independently of the ion beam. Therefore, the electron current needs to be controlled in accordance with the ion beam. On the other hand, since the electron beam is similarly irradiated onto the surface of the sample to which no ion beam is irradiated, a corresponding negative charging is caused by the electrons. Consequently, there is a problem in that, for example the insulating structure of the sample is broken.
In addition, in the foregoing conventional ion beam irradiating apparatus, the emission coefficient of the secondary electrons from a sample varies, depending upon the sorts of ion and sample and the like. Consequently, there are problems, e.g., the moving energy of the electrons needs to be controlled in accordance with the variation in the emission coefficient, the control becomes complicated, and the like.
Further, there is also a problem that the electrons once injected into the ion beam are returned to the electron-beam emitting source because of the kinetic energy of the electrons themselves. On the other hand, since these electrons do not have the initial velocity component in the irradiating direction of the ion beam, a high energy is applied thereto in order to move the electrons toward the sample. Thus, there is also a danger that the electrons will collide with the sample and break it down.